A Multiple-Input Multiple-Output (MIMO) technique refers to a technique for improving data transmission/reception efficiency using multiple transmission antennas and multiple reception antennas, instead of employing one transmission antenna and one reception antenna. Recently, considerable research into the channel capacity of a multi-user MIMO system has been carried out so that multiple users may efficiently use spatial resources.
Upon using a single antenna, a receiving end receives data through a single antenna path, while upon using multiple antennas, the receiving end receives data through multiple paths. Accordingly, data transmission rate and transmission amount can be improved and coverage can be increased.
A wireless communication system using multiple antennas includes an open-loop multi-antenna system in which feedback information from a receiving end is not used and a closed-loop multi-antenna system in which feedback information from a receiving end is used. In the closed-loop multi-antenna system, a receiving end transmits feedback information about a channel state to a transmitting end and the transmitting end discerns the channel state through the feedback information, thereby improving the performance of the wireless communication system.
The closed-loop multi-antenna system uses a precoding scheme for minimizing an influence caused by a channel, in which the transmitting end processes transmission data using feedback information about channel environments received from the receiving end.
The precoding scheme refers to a scheme for raising a Signal-to-Noise Ratio (SNR) and includes a codebook based precoding scheme and a precoding scheme for quantizing and then feeding back channel information.
The codebook based precoding scheme refers to a scheme in which a receiving end selects a precoding matrix from a codebook shared previously by a transmitting end and the receiving end and feeds back an index of the selected precoding matrix to the transmitting end, and then the transmitting end modifies transmission data using the feedback precoding matrix. In other words, since feedback of all channel information functions as significant system overhead, a codebook is configured by quantizing the channel information for feedback, an index is allocated to each of precoding matrices included in the codebook, and the receiving end feeds back only the index to the transmitting end, thereby reducing overhead.
The codebook based precoding scheme according to the prior art includes Per User Unitary and Rate Control (hereinafter, “PU2RC”) and SIC-based Per User and Stream Rate Control (S-PUSRC).
PU2RC uses Fourier basis as a unitary matrix for precoding and uses a unitary matrix extended according to the number of transmission antennas as a precoding matrix.
A Dirty Paper Coding (DPC) scheme, which can reduce data interference generated from other users by previously eliminating, at a Base Station (BS), data of other users functioning as interference, provides maximum channel capacity in a MIMO system. However, it is difficult to implement the DPC scheme in an actual system due to much channel information demanded by a transmitting end and operation complexity. The PU2RC scheme, which is proposed in “Downlink MIMO for EUTRA” of 3rd Generation Partnership Project (3GPP) TSG RAN WG1 #44/R1-060335 for actual implementation, is a scheme for simultaneously allocating spatial resources to different users. According to this scheme, each user selects a precoding matrix and vector which can maximize channel transmission rate thereof from among a plurality of precoding matrices having an orthogonal basis and feeds back an index of the selected vector and a Signal to Interface plus Noise Ratio (SINR) to a BS.
The BS selects and transmits a precoding matrix and user for maximizing the sum of transmission rates based on the feedback information. Such a conventional PU2RC scheme is a technique for obtaining precoding gain using spatial multi-user diversity and multiple precoding matrices. However, since this technique determines a user and a precoding matrix in consideration of only single-cell environments, capabilities are deteriorated due to inter-cell interference in multi-cell environments.
Accordingly, in order to control inter-cell interference which may encounter in the multi-cell environments, a BS cooperative scheme or coordinated scheme has been discussed. Especially, for a cell boundary user that is subject to much interference from neighboring cells, the BS cooperative technique may be a good solving scheme for guaranteeing data transmission rate up to a given level or more.
Recently, although the BS cooperative technique has been dealt with, a method for solving an optimal problem when all BSs are aware of most channel information is proposed. A thesis “Base Station Cooperation for Multiuser MIMO: Joint Transmission and BS Selection” written by HongyuanZhang, HuaiyuDai, and QuanZhou discloses a DPC or joint Zero Forcing Beamforming (ZF-BF) in the case where all BSs know channel information.
In this case, however, since all BSs should be perfectly aware of channel information, application to an actual system is difficult in terms of feedback or operation complexity. A thesis “Receiver-Enhanced Cooperative Spatial Multiplexing with Hybrid Channel Knowledge” by Hilde Skjevling, David Gesbert, and Are Hjorungnes proposed a cooperative scheme between BSs under the assumption that a neighboring cell uses only statistical information as channel information and fully knows channel information of a served cell. However, since this proposal also assumes that a neighboring cell perfectly knows channel information of a served cell, actual application is difficult.
Furthermore, since a precoding matrix has conventionally been designed in consideration of only circumstances in which there is no power difference in reception signals between BSs, there may be elements which should be additionally considered when BS cooperation is applied in the other circumstances.
As described above, since the conventional PU2RC determines a user and selects a precoding matrix, by considering only a single cell, it is weak in inter-cell interference in multi-cell environments. A conventional precoding matrix generation method for multi-BS cooperation has been considered only when the numbers of antennas of BSs participating in cooperation are the same as the numbers of transmission layers. Furthermore, the conventional method cannot be effectively applied when there is power difference in reception signals between BSs.